I think he is 10th in the Open Era (so obviously lower down in the all time rankings). He is a great player though and will climb up the ladder considerably by the end of his career.
I rank players in the open era on their Slams Wins, Season end championship wins, Slam finals made and Master 1000 equivalents won. That levels the playing field, so that small inconsequential tournaments are left out. Then weight it using the currently used ATP points system - 2000 points for Slam win, 1400 points for Season end championship win (average between unbeaten winner and winner who lost one round robin match), 1200 points for Slam Finalist and 1000 points for Masters 1000 equivalent wins. That seems an objective way to compare. On that basis Djokovic is sitting at 10th in the Open era currently (behind Federer, Lendl, Sampras, Nadal, McEnroe, Borg, Connors, Agassi, Becker and ahead of Edberg and Wilander).
Here is the thread with the weighted Open era rankings: